Device for generating terahertz radiation, and a semiconductor component

ABSTRACT

The invention relates to a device for generating terahertz (THz) radiation comprising a short pulse laser ( 1 ) with mode coupling to which a pump beam ( 3 ) is supplied, and comprising a semiconductor component equipped with a resonator mirror (M 4 ). This semiconductor component serves to derive the THz radiation based on incident laser pulses. The resonator mirror (M 4 ), preferably a resonator end mirror, is provided with a semiconductor layer ( 8 ), which is partially transparent to the laser radiation of the short pulse laser ( 1 ), whose absorption edge is lower than the energy of the laser radiation of the short pulse laser ( 1 ) and on which the electrodes ( 9, 10 ) that can be connected to a bias voltage source are placed in order to generate and radiate the THz radiation in the electric field.

The invention relates to a device for generating terahertz (THz)radiation comprising a short pulse laser with mode locking to which apump beam is supplied, and a semiconductor component including aresonator mirror, which semiconductor component simultaneously isdesigned for deriving the THz radiation on the basis of impacting laserpulses.

Furthermore, the invention relates to a semiconductor componentincluding a resonator mirror to be used in a laser, which resonatormirror is adapted to enable mode-locked operation of the laser, whereinthe semiconductor component simultaneously is designed to generateterahertz (THz) radiation on the basis of impacting laser pulses.

Electromagnetic radiation in the terahertz range (10¹¹ Hertz to 10¹³Hertz), i.e. in the form of continuous waves just as in the form ofpulses, would be usable with great advantage e.g. in spectroscopy, butalso in other fields, e.g. in future computers. Various proposals havealready been made for the generation of such a terahertz radiation, ase.g. in Sarukura et al., “All-Solid State, THz Radiation Source Using aSaturable Bragg Reflector in a Femtosecond Mode-Locked Laser”, Jpn. J.Appl. Phys., Vol. 36, Part 2, No. 5A, 1 May 1997, pp. L560-L562. In thisarticle, the use of a mode-locked laser for generating short laserpulses in connection with a semiconductor mirror, a saturable Braggreflector (SBR element), has been described which comprises a quantumwell for generating terahertz radiation. The SBR element is installedwithin the resonator of a mode-locked laser, wherein the impact angle ofthe laser beam approximately corresponds to the so-called Brewsterangle. In this manner, outcoupling of the terahertz radiation ispossible. What is disadvantageous, however, is that each laser pulseimpacts twice on the SBR element during its roundtrip in the resonator,whereby the terahertz radiation is radiated in four differentdirections. Thus, an efficient bundling and use of the generatedradiation is not possible, and only extremely low outputs of theterahertz radiation—in the range of nW—are achieved.

In a further article by Sarukura et al., “THz-radiation Generation froman Intracavity Saturable Bragg Reflector in a Magnetic Field”, Jpn. J.Appl. Phys. Vol. 37, No. 2A, 1 Feb. 1998, pp. L125-L126, a somewhatmodified arrangement of a SBR element in connection with a short pulselaser with mode locking is disclosed, wherein the SBR element is used asan end mirror of the laser resonator. There, the SBR element is mountedin the field of a permanent magnet, wherein the magnetic field controlsthe radiation pattern of the main lobes of the terahertz radiation tothus prevent capture of the radiation within the substrate of the SBRelement. With this arrangement of the SBR element as a resonator endmirror, an increase of the output of the terahertz radiation up to avalue of 0.8 μW was, in fact, obtained, yet a higher output would stillbe desirable for practical applications, apart from the fact that theprovision of a magnetic field is complex in practice.

A similar arrangement with an InAs(indium arsenide) mirror in a magneticfield is disclosed in Liu et al., “Efficient Terahertz RadiationGeneration from Bulk InAs Mirror as an Intracavity Terahertz RadiationEmitter”, Jpn. J. Appl. Phys. Vol. 39, Part 2, No. 4B, 15 Apr. 2000, pp.L366-L367. There, the impacting angle of the laser beam on the InAsmirror is very large, in the range of 85°. This mirror again is arrangedwithin the laser resonator, resulting in the already previouslymentioned disadvantage that each laser pulse meets the mirror twice andthat thus the terahertz radiation is generated in four directions. Whencarrying out experiments, the average output of the terahertz radiationachieved was in the range of 5 nW, with an average laser resonatoroutput of 4.5 W. A further disadvantage is that with the InAs mirror aseparate component is additionally introduced in the laser resonator.Besides, the arrangement of an SBR element in a magnetic field is alsoknown from the earlier article by Liu et al., “THz Radiation fromIntracavity Saturable Bragg Reflector in Magnetic Field withSelf-Started Mode-Locking by Strained Saturable Bragg Reflector”, Jpn.J. Appl. Phys., Vol. 38, Part 2, No. 11B, 15 Nov. 1999, pp. L1333-L1335.

Furthermore, a mode-locked laser with an SBR element is described in Liuet al., “High Average Power Mode Locked Ti:Sapphire Laser withIntracavity Continuous-Wave Amplifier and Strained Saturable BraggReflector”, Jpn. J. Appl. Phys., Vol. 38, Part 2, No. 10A, 1 Oct. 1999,pp. L1109-L1111.

From EP 606 776 A, furthermore a device for delivering terahertzradiation is known in which an superimposed arrangement of twoelectrodes on a substrate is provided, between which an LT-GaAs materialshall be provided. With the occurrence of laser pulses, the terahertzradiation is generated in the plane of the substrate, resulting intechnological disadvantages and a low robustness.

Another manner of generating terahertz radiation by using an antennawith a large aperture is described in Fattinger et al., “Terahertzbeams”, Appl. Phys. Lett. Vol. 54, No. 6, 6 Feb. 1989, pp. 490-492.Here, the generation of the terahertz radiation is based on a transientphotocurrent obtained by optically generated charge carriers which movein an electric field between two electrodes. The semiconductor materialused for the emitter typically has a high resistance, with the usefullife of the charge carriers being very short. A correspondingarrangement is also described in U.S. Pat. No. 5,729,017 A. It is alsoknown to use compounds such as GaAs (gallium arsenide) compounds, AlGaAs(aluminium-gallium-arsenide) compounds, LT-GaAs compounds and LT-AlGaAscompounds (LT—low temperature) for the semiconductor material in whichthe charge carriers are produced, cf. also Mitrofanov et al., “Thinterahertz detectors and emitters based on low temperature grown GaAs onsapphire”, Conference on Lasers and Electro-Optics (CLEO 2000).Technical Digest. Postconference Edition. TOPS Vol. 39; IEEE Cat. No.00CH37088. Opt. Soc. America, Salem, Mass., USA; May 2000; pp. 528-529.

The aforementioned low temperature semiconductor materials are appliedat low temperatures in the order of 200° C. to 500° C., and they arecharacterised by short recombination times of the photo charge carriers.

In particular, here it is also known that in case of LT-GaAs materialwith the light-induced transient terahertz radiation a recombinationtime of the charge carriers of a few ps or below 1 ps is attainable.

Departing from the known investigations, it is now an object of theinvention to provide a device and a semiconductor component,respectively, with which the generation of terahertz radiation by usinga mode-locked short pulse laser is efficiently enabled, wherein also theoutput of the terahertz radiation shall be substantially higher than inthe known arrangements and, preferably, shall also be controllable. Inparticular, for the terahertz radiation outputs in the range of mW shallbe rendered possible.

The arrangement according to the invention and of the initially definedtype is characterised in that the resonator mirror, preferably aresonator end mirror, is provided with a semiconductor layer which ispartially permeable for the laser radiation of the short pulse laser,the absorption edge of the semiconductor layer being below the energy ofthe laser radiation of the short pulse laser, and electrodes connectableto a bias voltage source being mounted thereon in a manner known per seso as to generate the THz radiation in the electric field and radiateit.

Correspondingly, the inventive semiconductor component of the initiallydefined type is characterised in that on the resonator mirror,preferably on a resonator end mirror, a semiconductor layer partiallypermeable for the laser radiation is provided, the absorption edge ofthe semiconductor layer being below the energy of the laser radiationand electrodes connectable to a bias voltage source being mountedthereon in a manner known per se so as to generate the THz radiation inthe electric field and radiate it.

Thus, the concept of the invention is generally based on combining thesemiconductor resonator mirror of the short pulse laser with asemiconductor layer with electrodes which also serve as antennas for theTHz radiation, and to generate the desired terahertz radiation in thissemiconductor layer on the resonator mirror by means of the laser beam.In doing so, the output of the terahertz radiation can be adjusted oreven modulated simply by means of the applied voltage, i.e. the appliedelectric field.

In mote detail, the intensity-rich laser pulse generates movable chargecarriers in the semiconductor material applied on the semiconductorresonator mirror; what is important in this context is, of course, thatthe energy of the laser beam be high enough so as to produce the chargecarriers, i.e. the energy of the laser radiation lies above theabsorption edge (that is that energy level starting from which electronsare lifted into the conduction band) of the semiconductor material,which therefore has to be chosen accordingly—depending on the type oflaser used, which can be done without any problems on the basis ofavailable semiconductor material data. Due to the electric fieldapplied, the thus generated electrons and the holes are brought out oftheir resting position, and depending on their charge, they will beaccelerated in opposite directions. The resultant polarisation leads toa return force, whereby plasma oscillation is obtained. This results ina transient photocurrent which generates the desired terahertz radiationwhich, for instance, is radiated through the resonator mirror. By meansof the applied voltage, the amount of the acceleration of the chargecarriers and, consequently, the intensity, or the output power,respectively, of the terahertz radiation can be controlled, or adjusted,respectively. For producing the terahertz radiation, the intensity-richoptic pulses of the short pulse laser are efficiently used, similar asin the suggestions made by Sarukura et al., as explained before, yet aprinciple of generating the terahertz radiation different therefrom isemployed, with the separate semiconductor layer on the resonator mirror,and with the generation of the movable charge carriers in thissemiconductor material by means of the laser pulses, similar asdescribed as such e.g. in the aforementioned U.S. Pat. No. 5,729,017 A.

Naturally, the semiconductor layer applied on the resonator mirror shallallow the laser radiation substantially to penetrate to the resonatormirror, wherein, however, a part of the energy of the laser radiation isused in the semiconductor layer for generating the charge carriers. Onthe other hand, the material of the resonator mirror, if the terahertzradiation is delivered throught the latter, must be chosen such that itwill be substantially permeable for the terahertz radiation generated.

Preferably, the resonator mirror is an end mirror, and in particular, itis formed by a saturable Bragg reflector (SBR element in short) knownper se. To avoid undesired saturation effects, it is advantageous if thesemiconductor layer is made of a semiconductor material with shortrecombination time for free electrons. The material of the semiconductorlayer suitably is chosen in adaptation to the material of the resonatormirror, it being suitable if the semiconductor layer is agallium-arsenide (GaAs) layer, in particular a low temperaturegallium-arsenide (LT-GaAs) layer. On the other hand, it may also beadvantageously provided that the semiconductor layer is analuminium-gallium-arsenide (AlGaAS) layer, in particular a lowtemperature aluminium-gallium-arsenide (LT-AlGaAs) layer. Suchsemiconductor materials can be grown on a Bragg reflector which in turnis made up of layers of aluminum-gallium-arsenide(Al-GaAs)/Alluminium-arsenide (AlAs), these layers being epitaxiallyapplied on a gallium-arsenide (GaAs) substrate. Advantageously,molecular beam epitaxy may be employed for applying the layers.

In order to bundle the generated terahertz radiation, it may furthermorebe suitable if a dielectric lens, e.g. of silicon (Si), gallium-arsenide(GaAs) or the like, is mounted as a beam control element for the emittedTHz radiation on the side of the resonator mirror that faces away fromthe electrodes.

In particular, the electrodes are designed strip-shaped and arranged inparallel to each other, having a width of from 5 μm to a few 10 μm,wherein the distance between the electrodes may be from 30 μm up to afew mm. Typically, the distance between the electrodes is larger thanthe diameter of the laser beam, at least the dimensions should be chosensuch that the intensity center of “gravity” of the beam cross-section ofthe laser beam is located between the electrodes. The electrodes may memade e.g. of metal, such as gold, aluminium, chromium,platinum-gold-layer systems or titanium-gold-layer systems, yet it isalso possible to form the electrodes of a doped semiconductor material,with the semiconductor material electrodes in turn being connected bymetallic contacts.

With such electrodes or antenna elements for generating the terahertzradiation and the aforementioned dimensions and distances, respectively,bias voltages in the order of 150 volts and more, practically even up to400 volts, may be applied so as to generate the electric field. Thelimit is given by the breakdown voltage in the semiconductor material.Preferably, the bias voltage source is adapted to deliver variable biasvoltages for adjusting the intensity of the THz radiation and/or formodulating the THz radiation.

In the following, the invention will be further explained by way ofpreferred exemplary embodiments illustrated in the drawings to which,however, it shall not be resticted. Therein,

FIG. 1 shows a diagram of a device having a short pulse laser withmode-locking and a semiconductor component used as resonator end mirror,which semiconductor component is adapted to generate terahertzradiation;

FIG. 2 schematically shows a side view of such a semiconductorcomponent;

FIG. 3 schematically shows a top view onto this semiconductor component,wherein also the application of a bias voltage to electrodes of thissemiconductor component is shown;

FIG. 4 shows the correlation between applied bias voltage and pulseamplitude of the generated THz radiation in a diagram;

FIG. 5 shows a typical pulse of the THz radiation vs. time in a diagram;

FIG. 6 shows an associated frequency spectrum of the THz radiation; and

FIG. 7 shows a modified semiconductor component with associated circuitfor applying an electric bias voltage in a schematic diagrammaticrepresentation.

In FIG. 1, a short pulse laser 1 is schematically illustrated, in which,e.g., the per se known Kerr-lens mode locking principle is used forgenerating the short pulse.

According to FIG. 1, the short pulse laser 1 includes a laser resonator2 to which a pump beam 3, e.g. an argon laser beam, is supplied. For thesake of simplicity, the pump laser (Argon laser, e.g.) itself has beenomitted and belongs to the art.

After having passed a lens L1 and a dichroic mirror M2, the pump beam 3excites a laser crystal 4, a titanium:sapphire solid laser crystal(commonly termed Ti:S in short in the literature and also in thefollowing) in the instant example. The dichroic mirror M2 is permeablefor the pump beam 3, yet highly reflective for the Ti:S laser beam. Thislaser beam, the resonator beam, impacts on a laser mirror M1 and isreflected by the latter to a laser mirror M3 which also serves foroutcoupling. This laser mirror M3 again reflects the laser beam back tomirror M1, and from there the laser beam is reflected to laser mirrorM2, passing through the laser crystal 4 a second time. From there, thelaser beam then is reflected to a resonator end mirror M4 with asaturable Bragg reflector 5, termed SBR element in short hereinafter,whereby a per se common X-folded laser resonator 2 is formed. Via theoutcoupling mirror M3, the laser beam is coupled out, with possiblecompensation means being provided, a compensation platelet 6 as well asa mirror in thin film technique not further shown providing for adispersion compensation as well as taking care that no undesiredreflections will occur in the direction of the laser resonator 2. Thelaser beam obtained in the manner described in the laser resonator 2 isdenoted by 7 in FIG. 1.

The laser crystal 4 is a plane-parallel body which is opticallynon-linear and forms a Kerr element which has a greater effectiveoptical thickness for higher field strengths of the laser beam 7, yet aslighter effective thickness if the field strength, or intensity,respectively, of the laser beam 7 is lower. This per se known Kerreffect is then used for self-focussing of the laser beam 7, i.e. thelaser crystal 4 forms a focussing lens for the laser beam 7.

In the exemplary embodiment illustrated, the saturable Bragg reflector 5is used for mode-locking in per se conventional manner.

The mirrors M1, M2 are made in per se known thin film technique, i.e.they are each designed with many layers which fulfil their function whenreflecting the ultra-short laser pulse which has a large spectralbandwidth. The different wave length components of the laser beam enterto different depths into the layers of the respective mirror beforebeing reflected. By this, the different wave length components aredelayed on the respective mirror for different amounts of time; theshort-wave components will be reflected rather outwardly, the long-wavecomponents, however, will be reflected deeper within the mirror. Thismeans that the long-wave components will be temporally delayed relativeto the short-wave components. In this manner, a dispersion compensationis attained insofar as pulses of a particularly short time range(preferably in the range of 10 femtoseconds and below) have a widefrequency spectrum; this is due to the fact that the different frequencycomponents of the laser beam in the laser crystal 4 “see” a differentrefraction index, i.e. the optical thickness of the laser crystal 4 isdifferently large for the various frequency components, and thedifferent frequency components therefore will be differently delayedwhen passing through the laser crystal 4. This effect is counteracted bythe above-mentioned dispersion compensation at the thin film lasermirrors M1, M2.

As described above, this is a conventional set-up of a short pulse laserwith mode-locking, and a detailed description of the same therefore isnot required.

What is essential for the sought generation of terahertz radiation 14 isthat the resonator mirror M4 is equipped with additional means in aspecial way, as will be explained in more detail with reference to FIGS.2 and 3.

In detail, the resonator mirror M4 comprises a semiconductor layer 8 asa semiconductor component on the SBR element 5 proper (cf. also FIG. 2in addition to FIG. 1), which semiconductor layer 8 consists of asemiconductor material with a short recombination time for freeelectrons. On this semiconductor layer 8, two substantiallystrip-shaped, parallel-extending electrodes 9, 10 are applied which areconnected with terminals 11 and 12, respectively (cf. FIG. 3) forapplying a voltage U to the electrodes 9, 10. The distance between thestrip-shaped electrodes 9, 10 is denoted by D in FIG. 3 and is chosensuch that the impacting laser beam 7 with its beam cross-section 7′ (cf.also FIG. 3) comes to lie substantially between the electrodes 9, 10during operation—at least the intensity centre of gravity of the beamcross-section 7′ of the laser beam 7 should lie between the electrodes9, 10 so as to avoid unnecessary losses. This distance D may, e.g., befrom 30 μm up to a few mm. The strip-shaped electrodes 9, 10, in turnmay have a width B of e.g. from 5 μm up to a few 10 μm.

The SBR element 5 as mirror and saturable absorber is assembled in usualmanner from a plurality of dielectric layers which, however, are notfurther illustrated in the drawing and which are applied to a substratelikewise not further visible in detail. The substrate may be made of aconventional material which is substantially permeable forelectromagnetic radiation in the THz range, in particular 10¹¹ Hz to10¹³ Hz, and it serves as a carrier for the Bragg reflector. Aconventional gallium-arsenide(GaAs) substrate with high resistance isused, e.g., which carries layers of aluminium-gallium-arsenide, oraluminium-arsenide, respectively, which are epitaxially grown on thegallium-arsenide substrate. Of course, however, also other combinationsof semiconductor materials and dielectric materials are possible tobuild up the Bragg reflector, and also other conventional productionmethods (thin film techniques) may be used.

The semiconductor layer 8 forms a saturable absorber, and it consists,e.g. in the case of a Bragg reflector with aluminium-gallium-arsenide,or aluminium-arsenide layers, of a gallium-arsenide applied at lowtemperature, a so-called LT (low temperature) GaAs layer, which, e.g.,is applied by molecular beam epitaxy (MBE) and has a saturableabsorption at a wave length of, e.g., approximately 800 nm and arecombination time in the order of picoseconds. Another possible way isto use LT-AlGaAs for the semiconductor layer 8, if shorter laserwavelengths are used. The thickness of the semiconductor layer 8 ischosen with a view to the sought pulse energy, which is absorbed,wherein the function of the Bragg reflector 5 should not bedeteriorated. In one concrete exemplary embodiment, an LT-GaAs layerhaving a thickness of 326 nm was grown as semiconductor layer 8 at 220°C. on an AlGaAs/AlAs Bragg reflector structure with a GaAs quantum well.Then the semiconductor layer 8 was heat-treated in a manner known per seat 660° C. for 10 min, and subsequently electrodes 9, 10 oftitanium-gold were applied to the upper side of the semiconductor layer8. Alternatively, metal electrodes 9, 10 of aluminium, chromium,platinum-gold etc. could be used; the choice of the metal for theelectrodes 9, 10 is not critical.

The width B of the electrodes was 10 μm, and the distance D between theelectrodes was 50 μm. As a bias voltage U, a direct voltage of 150 voltswas applied to the thus obtained THz emitter.

For bundling the THz radiation 14 generated and to be delivered, acollimator-beam control element in the form of a dielectric lens 13 canbe applied to the rear side or outer side of the resonator mirror M4located opposite the electrodes 9, 10 and illustrated in dot-and-dashlines in FIG. 2 (i.e. at the rear side of the substrate of the SBRelement 5), this dielectric lens 13 bundling the THz radiation 14 in thedesired direction. As the material for this beam control element 13,high-resistance silicon, semiinsulating gallium-arsenide or sapphiremay, e.g., be used. Such a dielectric lens 13 was also present in thepreviously described practical exemplary embodiment.

In the case of the previously described practical exemplary embodiment,the resonator mirror M4 thus formed was attached as end mirror in thelaser resonator 2 of the short pulse laser with mode-locking, and theelectrodes 9, 10 were connected to an external voltage supply unit notfurther illustrated in FIGS. 2 and 3, respectively, for applying thebias voltage U. Due to the saturable absorber (GaAs quantum well) in thelaser resonator 2, a mode-locking was achieved independently of the biasvoltage at electrodes 9, 10. Without a bias voltage U at the electrodes9, 10, no measureable THz radiation could be detected, however, whenapplying the bias voltage U to the semiconductor layer 8 via theelectrodes 9, 10, there resulted a THz radiation, the intensity of whichincreased with the bias voltage.

In FIG. 4, the resultant, substantially linear correlation between thepulse amplitude of the THz radiation (in μV) and the applied biasvoltage (in V) is visible. The average radiation output was measuredwith a calibrated silicon bolometer, wherein with the present simpletest embodiment, already a value of 1.5 μW was obtained at an averageresonator output of 900 mW. The typical shape of the transient THzsignal which is generated in this manner in the resonator mirror M4,i.e. semiconductor component, is illustrated in FIG. 5. As can be seen,there is substantially one single narrow pulse, which means that abroadband signal is achieved. For this desired occurrence of a singlepulse (instead of several decaying pulses), the short recombination timeof the charge carriers in the semiconductor material is co-responsible,and this is particularly achieved in case of a low temperatureapplication of the semiconductor layer 8.

The waveshape of the THz pulse illustrated in FIG. 5 (the amplitude isshown in arbitrary units) was registered by means of an electroopticaldetector. In the frequency range there results a corresponding amplitudecourse (again with the amplitude in arbitrary units) as shown in FIG. 6.The frequency spectrum has a maximum at approximately 0.5 THz andextends to up to approximately 2.5 THz. This THz radiation isvoltage-controlled by means of the bias voltage U at the emitterelectrodes 9, 10, wherein also a modulation with frequencies of up to 50kHz (cf. the following explanations relating to FIG. 7) was tested.

The generated THz radiation 14 is indicated by an arrow and by dashedlines in FIG. 2 and by an arrow in FIG. 1.

Instead of metallic electrodes, also electrodes 9, 10 of highly dopedsemiconductor material, such as, e.g., gallium-arsenide layers, areconceivable for generating the THz radiation 14. Such an embodiment willbe advantageous if the beam cross-section 7′ of the laser beam 7 has alarger diameter than the distance between the electrodes 9, 10, andreflections on the metallic electrodes 9, 10 would impair the laseractivity. The electrodes 9, 10 then will be made e.g. in an etchingprocess (wet etching or dry chemical etching), and externally of theimpacting region of this laser beam 7, metallic contacts 9′, 10′ may beapplied to the electrodes 9, 10 of semiconductor material, asschematically illustrated in FIG. 7 at a modified resonator end mirrorM4.

The width of the strip-shaped electrodes 9, 10 may, as mentioned, quitegenerally be e.g. from 5 μm up to several 10 μm so as to keep low theentire resistance. The distance between the electrodes 9, 10 may be from10 μm or several 10 μm up to several mm. Here, the limit will bedetermined by the desired breakdown voltage, on the one hand, and by thebeam cross-section 7′ of the laser beam 7 on the THz emitter, on theother hand. Mostly, the distance between the electrodes 9, 10 will belarger than the diameter of the laser beam 7.

Besides, in FIG. 7 again the embodiment of the resonator end mirror M4with the SBR element 5 and the semiconductor layer 8 is visible.Furthermore, it is illustrated in FIG. 7 that the one electrode 10 isconnected to ground 15 via the metallic contact 10′, whereas the otherelectrode 9, via its metallic contact 9′, is connected to a signalsource 16 with variable frequency, with a high voltage ammplifier 17interposed. In this manner, the THz radiation generated (14 in FIGS. 1and 2) can be controlled in its intensity according to the frequency ofthe bias voltage U. Of course, circuits 18 known for frequency variationand not further visible in FIG. 7 can be used in connection with thesignal source, i.e. bias voltage source 16.

1. A device for generating terahertz (THz) radiation having a shortpulse laser with mode locking to which a pump beam is supplied, and asemiconductor component including a resonator-mirror, whichsemiconductor component simultaneously is designed for generating theTHz radiation on the basis of impacting laser pulses, characterised inthat the resonator mirror (M4) is provided with a semiconductor layer(8) which is partially permeable for the laser radiation of the shortpulse laser (1), the absorption edge of the semiconductor layer beingbelow the energy of the laser radiation of the short pulse laser (1),and electrodes (9, 10) connectable to a bias voltage source beingmounted thereon so as to generate the THz radiation in the electricfield and radiate it.
 2. A device according to claim 1, characterised inthat the resonator mirror (M4) is a resonator end mirror.
 3. A deviceaccording to claim 1, characterised in that the resonator mirror (M4) isa saturable Bragg reflector (5).
 4. A device according to claim 1,characterised in that the semiconductor layer (8) is made of asemiconductor material with short recombination time for free electrons.5. A device according to claim 1, characterised in that thesemiconductor layer (8) is a gallium-arsenide(GaAs) layer.
 6. A deviceaccording to claim 5, characterised in that the semiconductor layer (8)is a low temperature gallium-arsenide (LT-GaAs) layer.
 7. A deviceaccording to claim 1, characterised in that the semiconductor layer (8)is an aluminium-gallium-arsenide (AlGaAs) layer.
 8. A device accordingto claim 7, characterised in that the semiconductor layer (8) is a lowtemperature aluminium-gallium-arsenide (LT-AlGaAs) layer.
 9. A deviceaccording to claim 1, characterised in that a dielectric lens (13) forthe emitted THz radiation is mounted on the side of the resonator mirror(M4) that faces away from the electrodes (9, 10).
 10. A device accordingto claim 9, characterised in that the dielectric lens (13) is made of amaterial selected from the group consisting of silicon, gallium-arsenide(GaAs) or the like.
 11. A device according to claim 1, characterised inthat the strip-shaped, parallel electrodes (9, 10) are spaced at adistance (D) of from 30 μm up to a few mm from each other.
 12. A deviceaccording to claim 1, characterised in that the strip-shaped electrodes(9, 10) have a width (B) of from 5 μm up to a few 10 μm.
 13. A deviceaccording to claim 1, characterised in that the electrodes (9, 10) aremade of metal.
 14. A device according to claim 13, characterised in thatthe electrodes (9, 10) are made of a metal selected from the groupcomprising gold, aluminium, chromium, platinum-gold-ortitanium-gold-layer systems.
 15. A device according to claim 1,characterised in that the electrodes (9, 10) are formed by dopedsemiconductor material electrodes connected with metallic contacts. 16.A device according to claim 1, characterised in that at least theintensity centre of gravity of the beam cross-section (7′) of the laserbeam (7) is located between the electrodes (9, 10).
 17. A deviceaccording to claim 1, characterised in that the bias voltage source (16)is adapted to deliver variable bias voltages.
 18. A semiconductorcomponent including a resonator mirror to be used in a laser, whichresonator mirror is adapted to enable mode-locked operation of thelaser, wherein the semiconductor component simultaneously is designed togenerate terahertz(THz) radiation on the basis of impacting laserpulses, characterised in that on the resonator mirror (M4), asemiconductor layer (8) partially permeable for the laser radiation (7)is provided, the absorption edge of the semiconductor layer being belowthe energy of the laser radiation (7) and electrodes (9, 10) connectableto a bias voltage source being mounted thereon in a manner known per seso as to generate the THz radiation in the electric field and radiateit.
 19. A semiconductor component according to claim 18, characterisedin that the resonator mirror (M4) is a resonator end miror.
 20. Asemiconductor component according to claim 18 characterised in that theresonator mirror (M4) is a saturable Bragg reflector (5) known per se.21. A semiconductor component according to claim 18, characterised inthat the semiconductor layer (8) is made of a semiconductor materialwith short recombination time for free electrons.
 22. A semiconductorcomponent according to claim 18, characterised in that the semiconductorlayer (8) is a gallium-arsenide(GaAs) layer.
 23. A semiconductorcomponent according to claim 22, characterised in that the semiconductorlayer (8) is a low temperature gallium-arsenide (LT-GaAs) layer.
 24. Asemiconductor component according to claim 18, characterised in that thesemiconductor layer (8) is an aluminium-gallium-arsenide (AlGaAs) layer.25. A semiconductor component according to claim 24, characterised inthat the semiconductor layer (8) is a low temperaturealuminium-gallium-arsenide (LT-AlGaAs) layer.
 26. A semiconductorcomponent according to claim 18, characterised in that a dielectric lens(13) e.g. made of silicon, gallium-arsenide (GaAs) or the like, for theemitted THz radiation is mounted on the side of the resonator mirror(M4) that faces away from the electrodes (9, 10).
 27. A semiconductorcomponent according to claim 26, characterised in that the dielectriclens (13) is made of a material selected from the group consisting ofsilicon, gallium-arsenide (GaAs) or the like.
 28. A semiconductorcomponent according to claim 18, characterised in that the strip-shaped,parallel electrodes (9, 10) are spaced at a distance of from 30 μm up toa few mm from each other.
 29. A semiconductor component according toclaim 18, characterised in that the strip-shaped electrodes (9, 10) havea width of from 5 μm up to a few 10 μm.
 30. A semiconductor componentaccording to claim 18, characterised in that the electrodes (9, 10) aremade of metal, e.g. gold, aluminium, chromium, platinum-gold- ortitanium-gold-layer systems.
 31. A semiconductor component according toclaim 30, characterised in that the electrodes (9, 10) are made of metalselected from the group comprising gold, aluminium, chromium,platinum-gold- or titanium-gold-layer systems.
 32. A semiconductorcomponent according to claim 18, characterised in that the electrodes(9, 10) are formed by doped semiconductor material electrodes connectedby metallic contacts.
 33. A semiconductor component according to claim18, characterised in that at least the intensity centre of gravity ofthe beam cross-section (7′) of the laser beam (7) is located between theelectrodes (9, 10).